Unimolecular decay pathways of state-selected CO[sub 2][sup +] in the internal energy range of 5.2–6.2 eV: An experimental and theoretical study.

The vacuum ultraviolet pulsed field ionization (PFI)-photoelectron (PFI-PE) spectrum of CO[sub 2] has been measured in the energy region of 19.0-20.0 eV. The PFI-PE vibrational bands resolved for CO[sup +, sub 2] are overwhelmingly dominated by the origin band along with weak vibrational bands corresponding to excitations of the v[sup +, sub 1] (symmetric stretching), v[sup +, sub 2] (bending), and v[sup +, sub 3] (antisymmetric stretching) modes. The simulation of the rotational contour resolved in the origin PFI-PE band yields a value of 19.3911±0.0005 eV for the ionization energy of CO[sub 2] to form CO[sup +, sub 2](C [sup 2]&Zigma;[sup +, sub g])./ A PFI-PE peak is found to coincide with each of the 0 K dissociation thresholds for the formation of O[sup +]([sup 4]S) + CO(X [sup 1]&Zigma;[sup +]) and CO[sup +] (X ²&Zigma;[sup +] ) +O(³P). This observation is tentatively interpreted to result from the lifetime switching effect, arising from the prompt dissociation of excited CO[sub 2] in high-n (n ≥ 100) Rydberg states prior to PFI. We have also examined the decay pathways for state-selected CO[sup +, sub 2] in the internal energy range of 5.2-6.2 eV using the PFI-PE-photoion coincidence scheme. The coincidence TOF data show unambiguously the formation of O[sup +]([sup 4]S) + CO(X 1&Zigma;[sup +]; v″ = 0,1) and CO[sup +](X ²&Zigma;[sup +] ; v[sup +] = 0,1) + O(³P). Analysis of the kinetic energy releases of fragment ions suggests that the dissociation of excited CO[sup +, sub 2] involved is nonstatistical and proceeds with an impulsive mechanism. Potential energy functions (PEFs) for the CO[sup +, sub 2] (C ²&Zigma;[sup +, sub g]) state and the lowest quartet states of CO[sup +, sub 2], together with their spin-orbit interactions, have been calculated using the complete active space self-consistent field and internal contracted multireference configuration... [ABSTRACT FROM AUTHOR]